Each gene has two alleles....... one dominant and one ressessive. But i want to know what are the structural differences between the two that makes former dominant n later ressessive. As per me both are made of some DNA makeup, then where does difference actually lie. How is it decided that this allele will be ressessive n that be dominant ?

It's a matter of gene expression. The phenotype is the expression of the gene. The pathway to expression involves various biochemical interactions. For whatever reason, the expression of one allele overrides the expression of the other. In some instances, it is because one allele isn't actually expressed, leaving the organism with a default phenotype.The dominant/recessive/codominant description can vary from the microbiological level to the organismal level.

I asked that question too. Maybe professors need to explain this more often. It only took me several sentences!

1) each gene can have from one to many alleles2) it's not only about expression. The difference can be anywhere from transcription to protein post-translational modifications. There can be mutation in the promotor changing significantly expression. If the binding of ribosome will be impaired, there will be much less of protein made. Single point mutations can lead to differences in binding of miRNAs. On the protein level, there are several options, you can have premature STOP codon and the resulting peptide is too short to do its work. Or you can have change of a crucial residue to turn off your protein, like in case of Asp to Asn mutations in receptor which impair its ability to transfer phosphates and thus signal the binding. Or you could change the post-translational modification and thus mis-localizing the protein or impairing its binding to partners or simply changing its activity.

Yes, it is possible that there are more than two different alleles for a gene. The most popular example, that comes into my mind, is blood types (AB0-system). But it is not possible that more than one is recessive to all the others - what would happen if two different recessive genes are inherited? The same is true for dominant alleles. Therefore there has to be a mixture of co-dominant, incomplete dominant, recessive and dominant relationships between the alleles.Of course, all of them could be co-dominant or incomplete dominant to each other.